A patchset to extend tmpfs to MAX_LFS_FILESIZE by abandoning its peculiar

A patchset to extend tmpfs to MAX_LFS_FILESIZE by abandoning its peculiar
swap vector, instead keeping a file's swap entries in the same radix tree
as its struct page pointers: thus saving memory, and simplifying its code
and locking.

This patch:

The radix_tree is used by several subsystems for different purposes.  A
major use is to store the struct page pointers of a file's pagecache for
memory management.  But what if mm wanted to store something other than
page pointers there too?

The low bit of a radix_tree entry is already used to denote an indirect
pointer, for internal use, and the unlikely radix_tree_deref_retry() case.
 Define the next bit as denoting an exceptional entry, and supply inline
functions radix_tree_exception() to return non-0 in either unlikely case,
and radix_tree_exceptional_entry() to return non-0 in the second case.

If a subsystem already uses radix_tree with that bit set, no problem: it
does not affect internal workings at all, but is defined for the
convenience of those storing well-aligned pointers in the radix_tree.

The radix_tree_gang_lookups have an implicit assumption that the caller
can deduce the offset of each entry returned e.g.  by the page->index of a
struct page.  But that may not be feasible for some kinds of item to be
stored there.

radix_tree_gang_lookup_slot() allow for an optional indices argument,
output array in which to return those offsets.  The same could be added to
other radix_tree_gang_lookups, but for now keep it to the only one for
which we need it.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>